The internal combustion engine (i.e., an engine, hereinafter) installed in a vehicle uses, for the most part, a knock control system for detecting knocking of the engine. The knock control system determines whether the engine is knocking or not based on the output signal of a knock sensor (e.g., a vibration sensor), and either advances or retards the ignition timing. That is, the ignition timing is advanced for the improvement of the torque when knocking is not detected, and the ignition timing is retarded for preventing the knocking when the knocking is detected.
In recent years, a knock detection method for detecting a small knock is desired for the improvement of combustion efficiency of the engine. For instance, according to, for example, Japanese Patent Laid-Open No. 2006-177259 (JP '259), a knock detection method is disclosed for accurately detecting a knock that is smaller than a noise vibration (i.e., a vibration caused by a factor other than the knock). Such knock detection is based on a comparison result that compares a predetermined ideal knock wave form with an actually-output sensor signal wave form from a knock sensor.
The knock detection method based on the knock wave form described above needs to quantize (i.e., digitize) the sensor signal in an accurate manner for the accurate detection of knocking. One of such detection method having such accuracy may be a method using a high resolution A/D converter. Conventionally, a ΔΣ type A/D converter is known for its high resolution at a comparatively low cost.
Further, for more accurate quantization of the sensor signal that changes its amplitude depending on the engine operation state, use of a variable amplification power (i.e., switching of amplification magnitudes) depending on the engine operation state at a moment of sensor signal detection is effective. That is, an amplification circuit having the variable amplification power may be used in combination with a middle-to-high resolution A/D converter, which is less expensive than the high resolution A/D converter, for the improvement of the knock determination accuracy at a relatively-lower cost.
In such case, the higher the amplification power of the variable-amp-power amplification circuit is, the more detailed the acquired information becomes. However, if the amplification power of the amplification circuit is too high, the output signal from the amplification circuit is saturated, and may deteriorate the accuracy of the knock determination.
To prevent such deterioration, Japanese Patent Laid-Open No. H05-340331 (JP '331) discloses a method that switches an amplification power, i.e., a gain, by having a large knock determination means for determining a large knock in addition to a normal knock determination means, and by utilizing a determination output from such large knock determination means.
Further, according to a method disclosed Japanese Patent Laid-Open No. H09-229823 (JP '823), a saturation of the signal from the amplification circuit is prevented by comparing the sensor output with a preset value and by inputting to the A/D converter the signal from a smaller-amp-power amplification circuit in a switching manner.
When the amplification power is switched in a period outside of the knock determination period, as disclosed in JP '331, it is important to quickly adjust/switch the amplification power to the change of the sensor signal amplitude in each of the knock determination period, for the accurate detection of a small knock.
Conventionally, the switching of the amplification power has been performed based on the engine rotation speed, the statistics of the amplitude of the past sensor signals, and the like. However, in such manner, the adjustment of the amplification power may not suitably be performed in response to the change of the sensor signal amplitude without delay, when the sensor signal amplitude drastically changes in each of the knock determination periods. That is, for example, when one ignition causing a large knock is followed by another ignition causing a small knock.
Therefore, the amplification power needs to have a smaller value, which includes a margin for accommodating a drastic change of the sensor signal amplitude, i.e., for preventing the saturation of the sensor signal output from the amplification circuit As a result, the accurate detection of the small knock becomes difficult, leading to the deterioration of the knock determination accuracy. Similar determination accuracy problem is also observed in a pre-ignition determination.